Lowboy trailers are often used to transport heavy machinery from one work site to another. These trailers are typically flatbed carriers with a gooseneck connection to a tractor. This type of trailer is commonly called a “lowboy” because the trailer is characterized by a low elevation of the trailer's deck. The low deck elevation enables both heavy and dimensionally oversized loads to be moved onto the trailer by way of a ramp that is at a relatively low incline because of the low elevation of the trailer deck.
However, even with the low elevation of the trailer deck that gives the trailer a characteristic look and its “lowboy” name, loading equipment onto the trailer is difficult if the equipment is driven up the ramp and up over onto the lowboy trailer, which is typically done when the equipment is self-powered. Examples of self-powered machines are any type of earth moving equipment that is mounted to a chassis with tracks or tires driven by an engine.
Lowboy trailers are typically designed to be loaded from one of the front, rear or sides of the trailer. In each of these cases, however, loading heavy earth moving equipment onto the trailer by driving the equipment up a ramp and onto the deck of the trailer creates an extreme force on both the edge of the trailer and the undercarriage of the earth moving equipment as the entire weight of the earth moving equipment transfers to the trailer at the trailer's edge during the equipment's movement up the ramp and over onto the trailer.
One solution to this problem of severe forces on the edges of the trailer is to lower the trailer to the ground during loading in order to stabilize the trailer while it experiences the large and dynamic forces caused by the equipment moving up a ramp and on to the deck of the trailer. Lowboys are known that are lowered at either their front or rear ends for equipment loading.
An example of a lowboy whose front is lowered is shown in U.S. Pat. No. 7,287,947 to Smith. For front-loaded lowboys, the tractor hitching apparatus or gooseneck is unhooked from the trailer chassis proper for loading and then rehooked up to the trailer chassis proper after the trailer chassis is loaded, making the loading process both cumbersome and time consuming. After the machinery to be hauled is loaded, the trailer hitch or gooseneck must be reattached to the front of the trailer chassis by the tractor and then the front of the trailer chassis must be hydro-mechanically elevated for transport.
Lowboys designed to load from the rear of the trailer do not require unhooking of the hitching apparatus. Examples of rear-loading lowboys are shown in U.S. Pat. No. 3,811,697 to Armstrong, U.S. Pat. No. 4,077,643 to Bates and U.S. Pat. No. 6,113,338 to Smith. These examples employ various apparatus and techniques for lowering the elevation of the rear of the trailer so that it is supported by the ground. The remaining difference in elevation between the ground and the deck of the trailer is bridged up and over by a ramp. In these examples, the trailer wheel assemblies are supported by axles extending from the sides of the trailer. Linkages and hydraulics move the wheel assemblies up and down relative to the trailer chassis in order to lower and raise the rear of the trailer.
However, often the weight of the machinery loaded on to the trailer is too much for a conventional wheel and axle assembly where the wheels and axles of the assembly extend cantilevered from the sides of the trailer for supporting the wheels and tires as in the Armstrong, Bates and Smith patents. The weight of the load carried by the trailer creates large moment arm torques on the cantilevered axles because of the outboard positions of the wheels and tires. The stresses on the axle created by these large moment arms makes it difficult for such trailers to carry the heaviest of loads such as the largest earth moving equipment.
To address the problem of the large moment arm torque and associated stresses on the axle, lowboy trailers are known that have wheel and tire assemblies at the rear of the trailer such that the axle or axles do not extend laterally beyond the sides of the trailer. This arrangement can reduce the moment arm about the axle. But the location of the axle and wheel assemblies obstructs access to the rear trailer edge for loading.
For this type of lowboy trailer, mechanical assemblies are known that enable the rear axle and wheel assemblies to be moved laterally out of the way of the rear of the trailer for loading and unloading. Hydraulic systems are employed to move the rear axle and wheel assemblies to positions outboard of the rear of the trailer and to also lower the trailer rear elevation so that it is as close to the ground as possible. Often though a ramp is still used to bridge the remaining elevation between the ground and the trailer deck.
An example of such a trailer is illustrated in U.S. Pat. No. 6,729,828 to Bosich, which describes a lowboy trailer with a chassis whose rear wheels are mounted on two axles attached to the chassis by way of pivoting support arms. In their transport position, the rear wheels are positioned at the rear of the chassis. The support arms pivot about two axes so that the wheels are raised vertically and rotated outwardly. Raising the rear wheels lowers the rear of the trailer chassis to the ground, which positions the rear of the trailer to receive a load. Rotating the wheels outwardly clears the lowered rear of the trailer to accommodate trailer loading for transport.
When the wheels of the Bosich trailer are in their positions for transporting a load, they are fastened together. The fastening of the two swing axles rigidly together prevents the wheels from migrating away from their transport position as the trailer moves, however, this prevents any floating of the rear wheels as the terrain undulates. The fastening is done hydro-mechanically, slowing the transition between loading and transport positions.